IRIS spacecraft has revealed several transient small scale phenomena
in the solar atmosphere
such as
UV bursts ([1]}), IRIS bombs or IBs ([2]}; [3]}; [4]}; [5]}), explosive events ([6]}; [7]}; [8]}; [9]}; [10]}) blow jets ([11]})
and bidirectional outflow jets ([10]}).
UV bursts are very tiny bright points with a bright core less than 2\(\mbox{$~\!\!^{\prime \prime }$}\) . Their lifetime is short (\(\approx \)  10 s) but with possibly recurrent enhancements during one hour giving the impression of
flickering ([13]}). With IRIS instrument the chromospheric C II and Mg II lines are frequently observed in the UV bursts and mainly in the quiet chromosphere as well as in solar flares and jets ([14]}; [15]}). They are optically-thick lines and need a radiative transfer approach to determine the physical quantities of plasma.
The Mg II h and k resonance lines in the quiet Sun are formed over a wide range of chromospheric heights. They usually appear as doubly peaked profiles with a central reversal.
IRIS spectral data allow to make many progresses on the plasma diagnostics in flares.
[16]} and [17]}
recently discussed the emission of chromospheric lines as observed in solar flares. They said about these lines that: “They appeared as redshifted, single-peaked profiles, however some pixels present
a net blue asymmetry”. The blue asymmetry can be explained by down-flowing plasma absorbing the red peak emission and not by strong blueshift emission
([18]}). IRIS spectroscopic and imaging observations of jets reveal bidirectional outflows (extended wings in chromospheric and transition line profiles) in transition region lines at the jet base, implying explosive magnetic reconnection processes ([19]}; [10]}; [21]}).
